1. Field of the Invention
The present invention relates to a plasma display panel (hereinafter also referred to in this specification as a PDP, simply) and a process for producing the same.
2. Description of Related Art
A PDP is well known as one of flat panel displays used for displaying images on a television set, a computer or the like. As an example, a general structure of a surface discharge type PDP with three electrodes is shown in FIG. 10. This PDP has a front plate 50 and a rear plate 60 which are opposed to each other (FIG. 10 shows an exploded view of the PDP while the front plate 50 and the rear plate 60 are located apart so that the internal structure of the PDP can be readily understood). The front plate 50 is made by forming on a glass substrate 51, parallel display electrodes 52 in pairs, a dielectric layer 53, and a protective layer 54, successively. The display electrode 52 is generally composed of a transparent electrode 52a and a bus electrode 52b. In the rear plate 60, on the other hand, address electrodes 62 perpendicular to the display electrodes 52 and ribs 63 locating between the address electrodes 62 are formed on a glass substrate 61, and phosphor layers 64a, 64b, 64c respectively emitting a light of red color (R), green color (G), and blue color (B) are applied to regions between the ribs 63. The front plate 50 and the rear plate 60 are disposed to be opposed to each other, and a space formed therebetween is filled with a discharge gas. The space filled with the discharge gas is a discharge space, and a discharge cell is formed at each of intersections of the display electrodes 52 and the address electrodes 62. A voltage is applied between a certain pair of the display electrode 52 and the address electrode 62 to cause address discharge, and therefore to accumulate wall charge in a certain cell. Then, a voltage is applied between a pair of display electrodes 52 to cause display discharge at the cell where the wall charge is accumulated, so that an ultraviolet ray is generated from the discharge gas. This ultraviolet ray irradiates to the phosphorous layers 64a, 64b, and 64c to realize the displaying of a color image.
It is generally desirable that the PDP has a higher luminous efficiency. For obtaining a higher luminous efficiency it is supposed that a discharge voltage of surface discharge (display discharge) is decreased to increase a power efficiency of the surface discharge, and for achieving this it is effective to increase an electric field intensity at a surface of the dielectric layer 53 of the front panel 50. As one of such measures, it is supposed to reduce a thickness of the dielectric layer 53 which covers the display electrodes 52. This measure has an additional advantage that a capacitance between the display electrodes 52 can be ensured even when an area of the discharge cell becomes smaller with a movement towards finer discharge cells.
It is hitherto general to use a glass material based on a lead oxide or a bismuth oxide as the dielectric layer. However, such dielectric layer may cause various problems when it is simply made to be thinner. For example, on forming the dielectric layer, particles are incorporated therein or gas bubbles generated therein, so that a withstand voltage of the dielectric layer may decrease or a transparency of the dielectric layer may decrease.
In order to avoid such problems, it is proposed to divide the dielectric layer into two layers, and to form these layers with different materials from each other. More specifically, as shown in FIG. 11, the first dielectric layer 53a is formed on the glass substrate 51 and the display electrodes 52 to cover them, and thereafter the second dielectric layer 53b covering the first dielectric layer 53a and consisting of a material different from the first dielectric layer is formed, and the protective layer 54 is formed on the second dielectric layer 53b, so that the front panel is prepared.
Such two-layered dielectric layer is disclosed in, for example, Japanese Patent Kokai Publication No. H11-195382. In this publication, the first dielectric layer is a layer of SiO2 formed by thermal decomposition of polysilazane in the atmospheric air, and the second dielectric layer is a layer of SiO2, Al2O3, or a compound of SiO2 and Al2O3 formed by a chemical vapor deposition (CVD) method. This publication also discloses that the second dielectric layer is a dielectric glass layer, which is prepared by firing.
In Japanese Patent Kokai Publication Nos. 2000-156168 and 2002-358894, the latter of which is a divisional application the former, the first dielectric layer is a dielectric glass layer of a high softening point, and the second dielectric layer is a dielectric glass layer of a low softening point, both prepared by firing.
In Japanese Patent Kokai Publication Nos. H11-54051 and 2003-7217, the latter of which is a divisional application the former, the first dielectric layer is a metal oxide layer (forming hydroxyl groups on its surface) obtained by a CVD method, and the second dielectric layer is a layer of a glass material based on a lead oxide or a bismuth oxide (having a permittivity not smaller than 10).